1. Field of the Invention
The present invention relates to an alternating current (AC) pulse arc welding method whereby the setting of welding conditions such as a wire feeding speed can be performed quickly and accurately.
2. Description of the Related Art
FIG. 7 illustrates a conventional AC pulse arc welder, which includes a wire feeding speed setting circuit FS, a negative current rate setting circuit RS, and a welding voltage setting circuit VS. The wire feeding speed setting circuit FS outputs a signal (wire feeding speed setting signal Fs) for setting the feeding speed of a welding wire 1. The negative current rate setting circuit RS outputs a signal (negative current rate setting signal Rs) for setting a negative current rate Ren to be described below with reference to FIG. 8. The welding voltage setting circuit VS outputs a signal (welding voltage setting signal Vs) for setting the average Vav of welding voltage Vw applied between the welding wire 1 and the base material (workpiece) 2.
The illustrated welder further includes a conventionally available power source 6, to which the above-mentioned setting signals Fs, Rs, and Vs are inputted. Based on these input signals, the power source 6 supplies welding current Iw and welding voltage Vw to be described below with reference to FIG. 8, while also supplying a wire feeding control signal Fc for controlling a wire feed motor WM.
The wire feed motor WM is connected to wire feed rollers 5. Upon actuation of the motor WM, the rollers 5 are rotated to feed the welding wire 1 through a welding torch 4. In welding operation, an electric arc 3 is generated between the welding wire 1 and the base material 2.
FIG. 8 shows the waveforms of a welding current Iw and a welding voltage Vw applied for AC pulse arc welding processes. The period from time t1 to time t2 is an electrode negative period Ten, during which the power source 6 is in negative polarity (EN). Thus, in the period Ten, a negative current In shown in FIG. 8(A) passes through the circuit, and a negative voltage Vn shown in FIG. 8(B) is applied between the welding electrode (i.e., the welding wire 1) and the base material 2.
The period from time t2 to time t4 is an electrode positive period Tep, during which the power source 6 is in positive polarity (EP). Thus, in the period Tep, a positive current Iw shown in FIG. 8(A) passes through the circuit, and a negative voltage Vw shown in FIG. 8(B) is applied between the welding wire 1 and the base material 2.
The electrode positive period Tep can be divided into two shorter periods (sub-periods): a peak period Tp (from t2 to t3) and a base period Tb (from t3 to t4). In the peak period Tp, as seen from the waveforms (A) and (B) in FIG. 8, relatively greater peak current Ip and peak voltage Vp are applied for transferring tiny globules of molten metal from the welding wire 1 to the base material 2. In the base period Tb, on the other hand, relatively smaller base current Ib and base voltage Vb are applied for preventing the growth of the molten metal globules.
The waveforms shown in FIG. 8 are periodic, each having a predetermined cycle Tf (from t1 to t4). The current and voltage application is repeatedly performed for the required welding processes.
In FIG. 8(A), the single dot chain line represents a welding current average Iav, which is the average (time average) of the absolute value of the welding current Iw. Likewise, in FIG. 8(B), the single dot chain line represents a welding voltage average Vav, which is the average (time average) of the absolute value of the welding voltage Vw.
In the following description, the absolute value of the negative current In is denoted simply by In. (Likewise, the absolute value of the negative voltage Vn is denoted simply by Vn.) The negative current rate Ren (%) is defined as follows:Ren=100×Ten×In/(Ten×In+Tp×Ip+Tb×Ib)
In the above definition, “Ten×In” represents the time integration of the negative current In for the pulse cycle Tf, and “Ten×In+Tp×Ip+Tb×Ib” represents the time integration of the welding current Iw for the same cycle Tf. Thus, the rate Ren represents a ratio of the total amount of the negative current to the total amount of the welding current for the cycle Tf.
As mentioned above, the wire feeding speed setting signal Fs is used for setting the feeding speed of the welding wire 1. In direct current (DC) pulse arc welding processes, the wire feeding speed is proportional to the welding current average Iav. Thus, to set the welding current average Iav means to set the wire feeding speed. The situations of AC pulse arc welding will be described later with reference to FIG. 9.
The negative current rate setting signal Rs is used for setting the negative current rate Ren defined above. Specifically, the negative current rate setting signal Rs adjusts the length of the electrode negative period Ten and/or the value of the negative current In, thereby varying the negative current rate Ren.
The welding voltage setting signal Vs can alter the pulse cycle Tf and the peak period Tp, for example, by feedback control. Thus, based on the signal Vs, it is possible to vary the welding voltage average Vav.
FIG. 9 illustrates the relationship between the negative current rate Ren and the welding current average Iav, with the wire feeding speed kept constant. More specifically, the figure illustrates three cases where the wire feeding speed is set to F1, F2 and F3 (F1>F2>F3), and each graph shows, for the relevant one of the three speeds, how the welding current average Iav varies with the change of the negative current rate Ren. As seen from the figure, in the AC pulse arc welding, the welding current average Iav varies as the negative current rate Ren changes, even when the wire feeding speed is constant.
In general, the welding current average Iav is proportional to the heat input to the base material, and the wire feeding speed is proportional to the amount of deposition. When thin metallic plates are welded to each other by DC pulse arc welding, the thickness of the plates determines an appropriate welding current average, and hence the wire feeding speed. For instance, a thin plate to be welded necessitates a small welding current average Iav, and a low wire feeding speed. Unfavorably, it is difficult to form proper weld beads with a low wire feeding speed and a small amount of deposition. In particular, when there is a gap between the metallic parts to be welded, the shortage of deposition is crucial.
By AC pulse arc welding, on the other hand, the setting of the wire feeding speed can be performed independently, to some extent, of the setting of the welding current average Iav. Thus, the above-described problem can be overcome by the AC pulse arc welding. In this sense, AC pulse arc welding is advantageous for the welding of thin base materials. AC pulse arc welding is often employed for the welding of aluminum parts, but is applicable to ferrous materials.
The above-described prior art is disclosed in JP-A-H05-92269, for example.
As described above, in the AC pulse arc welding, the change in the negative current rate Ren leads to the change in the relationship between the wire feeding speed and the welding current average Iav, and this fact can be utilized for performing proper welding of thin plates. In welding thin plates, two important factors to be considered are penetration and gap allowance. A proper penetration is formed by proper heat input to the base material. Regarding the gap allowance, the adjustable range of deposition should be sufficiently wide. In light of these, it is required that the welding current average Iav is adjusted to optimize the heat input to the base material, and that the negative current rate Ren is adjusted to optimize the wire feeding speed for attaining proper amount of deposition.
FIG. 10 shows the relationship between the negative current rate Ren and the heat input to the base material, with the welding current average kept constant. The measurement of the heat input to the base material was conducted with the use of: a welding wire made of aluminum alloy A5356 and having a diameter of 1.2 mm; and a base material made of aluminum A052 and having a thickness of 4 mm. The AC pulse arc welding with respect to the base material was performed for 30 seconds, the welding speed was 60 cm/min, and the welding current average Iav was kept at 100 A. As shown in the figure, when the welding current average Iav is constant, the heat input to the base material is generally constant, while the negative current rate is not constant. Due to the constant heat input, the penetration of the welding into the base material is generally constant. In the illustrated measurement, the wire feeding speed was adjusted so that the welding current average Iav remained constant, even when the negative current rate Ren changed.
FIG. 11 illustrates the relationship between the negative current rate Ren and the gap allowance of the base materials, with the welding current average kept constant. In the illustrated measurement, where lap welding was conducted, use was made of a welding wire having a diameter of 1.2 mm and made of aluminum alloy A5356. The base materials were plates having a thickness of 1.0 mm and made of aluminum alloy A5052. The welding speed was 60 cm/min, and the welding current average Iav was kept at 60 A. In the illustrated measurement, the maximum joint gap that allowed proper welding operation was determined. As shown in the figure, when the welding current average Iav was constant, the gap allowance becomes greater as the negative current rate Ren becomes greater. This is because, as the negative current rate Ren increases, the wire feeding speed becomes faster, and therefore the amount of deposition becomes greater. Accordingly, even a wide gap can be filled with a greater amount of molten metal. In this manner, a wide gap in the metallic parts to be connected can be filled by increasing the negative current rate Ren.
As seen from the above explanation with reference to FIGS. 10 and 11, the following is concluded. Proper heat input is determined by the thickness of base materials, the type of the joint, etc, and once the heat input is determined, a proper welding current average Iav is determined. At the same time, a proper wire feeding speed is determined by the gap at the joint, and the wire feeding speed in turn determines a proper negative current rate Ren. In summary, once the thickness of the base materials, the type of the joint, and the gap are determined, proper welding current average Iav and negative current rate Ren are determined.
In the conventional welding processes, as noted above with reference to FIG. 7, the wire feeding speed and the negative current rate Ren are to be set for performing welding. In the conventional manner, however, when the wire feeding speed setting signal Fs is adjusted for setting the welding current average Iav, the negative current rate Ren will inevitably change. Further, when the negative current rate setting signal Rs is adjusted, not only the negative current rate Ren but also the welding current average Iav will change. Namely, in the conventional welding system, the desired setting parameters (i.e., the welding current average Iav and the negative current rate Ren) fail to correspond to the actual setting parameters (i.e., the wire feeding speed and the negative current rate Ren) in a one-to-one relationship. As a result, the setting procedure becomes complicated, as noted above, and thereby taking much time. Further, the human operator needs to fully understand the above-described characteristics for conducting the setting procedure properly. This means that the human operator is required to have much experience and acquire the skill for conducting desired settings.